This invention generally relates to development of images and more specifically to electrophotographic development employing magnetic toner particles which can be applied from a magnetic brush to the electrostatic latent image without using a carrier material as is normally employed in most development systems.
The use of development systems for magnetic development without employing carrier materials has been suggested for example, in Wilson U.S. Pat. No. 2,846,333 which discloses the use of magnetic brush system to apply toner particles formed of ferrites and resin materials for developing electrostatic latent images. Although this process performs satisfactorily in most environments, the conductivity of the toner resulted in some instances in causing the electrostatic transfer to be somewhat difficult.
Kotz U.S. Pat. No. 3,909,258 also teaches the development of magnetic systems without carriers and more specifically there is described in this patent an electrostatic development process employing an inductive magnetic brush without carrier. A toner useful in the magnetic development process of the Kotz patent is described in Nelson U.S. Pat. No. 3,639,245 which teaches a dry toner particle having specific electric conductivity. This type of toner is prepared by blending magnetite with the resin and subsequently pulverizing the material to a small particle size. These particles are then mixed with conductive carbon black which is imbedded in the surface of the particle and small particles of SiO.sub.2 are mixed into the toner to improve flowability. One disadvantage of this type of toner although it is sufficient for most purposes, is that it does not transfer consistently from a photoconductive substrate to a plain bond paper thereby resulting in some instances in image quality of lower resolution that is normally expected. More specifically, the poor transfer efficiency results in low image density and causes photoreceptor cleaning problems.
Also of interest is U.S. Pat. No. 3,345,294 which discloses a development system employing a magnetic brush comprising magnetic carrier particles and magnetic toner, and in one embodiment the brush may be electrically biased. A problem encountered with this type of system is the difficulty in providing a toner that will both be charged properly triboelectrically, be held magnetically during development and continue to retain its charge after development in order that it will be able to transfer electrostatically. Many times magnetic type behaving toners are conductive enough that there is difficulty with charge exchange that occurs with the photoreceptor and further difficulty with powder clouds developing in the developing housing.
Accordingly, there is a need for magnetic toners suitable for use in one component magnetic development systems, such toners also being suitable for high speed development and having acceptable electrostatic transfer characteristics for transfer from a photoconductive surface to plain bond paper. A magnetic pigment coating which aids field dependence without introducing undesirable properties such as humidity sensitivity into the toner is also needed. It would also be desirable to have methods of rendering pigments hydrophobic. It is also important to have toner compositions to transfer to paper for example, they will adhere effectively by electrostatic forces in order that image disturbances which will cause blurring do not readily occur when the unfused image is processed prior to fixing.
In a copending application U.S. Ser. No. 792,636, filed May 2, 1977, on Single Component Magnetic Toner, the subject matter of this application which is hereby totally incorporated by reference, there is disclosed a coating of magnetic pigments with fatty acid prior to mixing with the resin and spray drying to form a toner which will produce field dependent toner material. However, there continues to be a need for additional field dependent type toners.